|The operating system is done.. there are many many reasons why this can happen, so unfortunately I can't answer this completely. |
However, the way to fix it is simple. If you still have the manufacture operating system cd, throw it in the drive and see what options it gives you. If you don't have the CD, you need to get a new operating system, and if you make it the same release as your current, you can try the repair feature, which might work in this case.
Lemme know if you need more help :)